From what I can tell purge absolutely depends on the user's strength.Hendricksen is able to use purge to kill grey demons but in turn zaratras, who seems to be stronger than hendricksen, couldn't scratch fraudrin with it. And when zaratras poured every ounce of his strength into the purge, presumably utilizing every bit of magic he had to the point the effort killed him even though he was already dead, all he managed to do was push fraudrin out. But fraudrin himself was not even scratched.
I am not sure elizabeth's "Let there be light" went straight past the defenses. If it did then the indura wouldn't have been able to resist it to begin with. In turn the implication was that their darkness was pushing back...
There absolute are magics whose effects remain the same regardless of PL but I would argue purge is not one of them. Even purification by goddesses, which while supereffective, does depend on the wielder's and target's power....